1. Field of the Invention
The present invention relates to the implantation of ions into semiconductor materials, and in particular, to a technique to reduce transient temperature effects in the semiconductor materials during implantation.
2. Description of the Prior Art
In ion implantation, a beam of ions with energies ranging from approximately 1 keV up to 3 MeV and beam currents varying from less than 1 microampere up to 100 milliamperes are incident upon a semiconductor substrate, typically silicon wafers. This results in incident beam powers of up to several kW. In addition, for implantation to be effective, a uniform distribution of the ions must be obtained in the semiconductor substrate. These two factors must be considered when designing an implant system to ensure that deleterious heating effects are reduced while simultaneously ensuring that a uniform ion distribution is obtained.
A number of cooling techniques have been developed to decrease the power density (W/cm.sup.2) incident upon the semiconductor (see for example, Robertson U.S. Pat. No. 3,778,626) and also to enhance the conduction cooling between the wafer and its mounting assembly (see the article by M. E. Mack in Handbook of Ion Implantation Technology, ed. J. F. Ziegler (Amsterdam, North Holland, 1992) and references cited therein).
In some systems, the beam is electrostatically scanned across the wafer at frequencies of several kHz. This approach permits approximately uniform distributions to be obtained and also results in transient temperatures that are equal across the wafer. However, this approach does not permit the use of high power densities to be employed during implantation.
In some systems, the implant dosimetry is controlled by a dual mechanically scanned system. Examples of such systems are described by Robertson in the aforementioned U.S. Pat. No. 3,778,626 and by Ryding in U.S. Pat. No. 4,234,797. In these systems, the wafers are mounted on the perimeter of a metal disk which rotates at high speeds, typically 500-1500 revolutions per minute (rpm). The spinning of the disk moves the wafers at high speed through the ion beam in one direction. Relative movement between the wafers and the ion beam in a direction transverse to said one direction is produced either by imparting a slow scanning movement to the ion beam across the wafers or imparting a slow scanning movement to the spinning disk. In the technique described by Ryding in U.S. Pat. No. 4,234,797, the beam current is sampled by a Faraday cup situated behind the spinning disk. There is a slot or number of slots cut into the disk along a radius or radii. During each revolution of the disk, a fraction of the beam passes through the slots and impacts into the Faraday cup. The sampled current is integrated and the accumulated charge drives the scan system in the direction transverse to the spinning direction. At the end of a scan, the slow scan direction is reversed and the beam again passes over the wafers. In this way, the dose is uniformly deposited into the wafer and the beam power is averaged over a large scan area to reduce the power density. However, this approach does not adequately address transient wafer heating effects.
A good description of the wafer heating effects, including transient effects, in a mechanically scanned ion implanter is found in the work by V. Benveniste described in Nucl. Instr. and Meth. B21 (1987) 366. His computer simulations demonstrate that in a mechanically scanned system of this type both the inner and outer edges of the wafer have significantly larger transient temperature excursions than the center of the wafer (see FIG. 2 of the aforementioned article by Benveniste.)
The mechanically scanned ion implanters of the prior art have been designed so as to maximize throughput of the implanters, and therefore provide a back-and-forth slow scan on top of a rotary motion fast scan. Whatever reasoning may be involved, the technical literature to date has not disclosed any use of a flyback technique, nor any suggestion of any benefit to be derived therefrom. Presumably it was thought that the inactivity of ion implantation during flyback was a substantial economic disadvantage.